BMW i3 Electric Car: ReX Range Extender Not For Daily Use?

The upcoming 2014 BMW i3 electric car is generating a lot of quiet excitement--especially for its optional "ReX" range extender, a tiny two-cylinder engine that fits under the rear deck to power a generator.

The i3 will be only the third high-volume dedicated battery electric car on the market, after the Nissan Leaf and Tesla Model S.

But those potential i3 owners need to understand a critical point: The BMW i3 may not operate like a Chevy Volt.

That is, its performance in range-extending mode may be more limited than when it is running on battery power.

One of the core principles behind the design of the Volt was that its performance should be identical whether in battery-powered or range-extending mode.

BMW says the range extender of the i3, on the other hand, is designed not for long-distance travel but purely as a short-term stopgap to get drivers to the next recharging location.

Will performance vary?

And while the BMW i3 is expected to have an electric motor producing 125 kilowatts (170 hp) of peak power output driving its rear wheels, the little range-extending inline twin (derived from a motorcycle engine) is likely to produce considerably less.

BMW has not yet given the precise displacement or power output for the ReX, nor the capacity of the generator it will drive.

But a good metric will be to see whether the generator develops at least half the output power of the traction motor.

The Volt's 83-hp 1.4-liter four-cylinder range extender runs a generator with a peak output of 55 kW (74 hp), which is half the peak power of the 111-kW (149-hp) traction motor that turns its wheels.

The Volt "buffers" power draw through the battery pack, so except under the most extreme circumstances, the performance of the Volt with its engine running a generator to power the traction motor is essentially the same as one running on battery power alone.

BMW's highest-performance 800cc vertical twin motorcycle engines carry ratings of 85 to 90 horsepower, so it's possible that a ReX range extender adapted from one of those engines could provide the same ratio.